An ultracentrifuge rotor may experience 600,000 g or higher forces which produce stresses on the rotor body which can eventually lead to rotor wear and disintegration. All ultracentrifuge rotors have a limited life before damage and fatigue of the material comprising the rotor mandates retirement from further centrifuge use.
Stress generated by the high rotational speed and centrifugal forces arising during centrifugation is one source of rotor breakdown. Metal fatigue sets into conventional rotors following a repeated number of stress cycles. When a rotor is repeatedly run up to operating speed and decelerated, the cyclic stretching and relaxing of the metal changes its microstructure. The small changes, after a number of cycles, can lead to the creation of microscopic cracks. As use increases, these fatigue cracks enlarge and may eventually lead to rotor failure. The stress on conventional metal body rotors may also cause the rotor to stretch and change in size. When the elastic limits of the rotor metal body have been reached, the rotor will not regain its original shape, causing rotor failure at some future time.
Conventional titanium and aluminum alloy rotors have a respectably high strength to weight ratio. Aluminum rotors are lighter weight than titanium, leading to less physical stress and a lower kinetic energy when run at ultracentrifuge speeds; however, titanium rotors are more corrosive resistant than aluminum. As the ultracentrifuge performance and speeds increase, the safe operating limits of centrifugation are reached by conventional dense and high weight metal rotors.
One attempt to overcome the design limitations imposed is indicated in U.S. Pat. No. 3,997,106 issued to Baram for a centrifuge rotor which is laminated and consists of two layers of different materials. Wires (24) are wound around a metal cover 8b which surrounds a central filler of chemically resistant plastics (See FIG. 3 of the '106 patent). The Baram '106 patent envisions greater chemical resistance and lower specific gravity rotors, which achieve optimum strength, by the use of a laminate manufacturing process. U.S. Pat. No. 2,974,684 to Ginaven (2,974,684) is directed to a wire mesh of woven wire cloth 6 for reinforcing a plastic material liner 7 for use in centrifugal cleaners (see FIGS. 2 and 3).
U.S. Pat. Nos. to Green (1,827,648), Dietzel (3,993,243) and Lindgren (4,160,521) have all been directed to a rotor body made from resin and fibrous reinforcement materials. In particular, Green '648 is fibre wound to produce a moment of inertia about the vertical axis greater than the moment of inertia about the horizontal axis through the center of gravity of the bucket so that the rotor bucket is stable at speeds of 7500 to 10,000 RPM (a relatively slow centrifuge speed by modern standards).
U.S. Pat. No. 4,468,269, issued Aug. 28, 1984 to the assignee of this application, discloses an ultracentrifuge rotor comprising a plurality of nested rings of filament windings surrounding the cylindrical wall of a metal body rotor. The nested rings reinforce the metal body rotor and provide strengthening and stiffening of the same. The rings are nested together by coating a thin epoxy coat between layers. U.S. Pat. No. 3,913,828 to Roy discloses a design substantially equivalent to that disclosed by the '269 patent.
None of the conventional designs provide maximum strength through ultracentrifuge speeds through the use of a material specifically designed to accommodate localized stress and resist rotor body fatigue. Conventional metal bodies, or reinforced metal body rotors, are subject to metal stress and fatigue failures during centrifugation.
What is needed is a rotor body of substantial strength, yet lighter in weight and capable of enduring increasingly higher loads and speeds. The body should resist stress and corrosion and be specifically designed to cope with localized stress.